On Thu, Oct 17, 2019 at 5:25 PM Vivek Goyal <vgo...@redhat.com> wrote: > > On Thu, Oct 17, 2019 at 01:23:57PM +0200, Miklos Szeredi wrote: > > On Thu, Oct 17, 2019 at 12:05 PM Stefan Hajnoczi <stefa...@redhat.com> > > wrote: > > > > > > On Wed, Oct 16, 2019 at 07:37:52PM +0900, Misono Tomohiro wrote: > > > > Hello, > > > > > > > > I test xattr operation on virtiofs using xfstest generic/062 > > > > (with -o xattr option and XFS backend) and see some problems. > > > > > > > > These patches fixes the two of the problems. > > > > > > > > The remaining problems are: > > > > 1. we cannot xattr to block device created by mknod > > > > which does not have actual device (since open in virtiofsd fails) > > > > 2. we cannot xattr to symbolic link > > > > > > > > I don't think 1 is a big problem but can we fix 2? > > > > > > Sorry, I don't know the answer. Maybe it would be necessary to add a > > > new O_SYMLINK open flag to open(2) so that fgetxattr()/fsetxattr() > > > operations can be performed. A kernel change like that would take some > > > time to get accepted upstream and shipped by distros, but it might be > > > the only way since the current syscall interface doesn't seem to offer a > > > way to do this. > > > > The real problem is that open() on a non-regular, non-directory file > > may have side effects (unless O_PATH is used). These patches try to > > paper over that, but the fact is: opening special files from a file > > server is forbidden. > > > > I see why this is being done, and it's not easy to fix properly > > without the ..at() versions of these syscalls. One idea is to fork() > > + fchdir(lo->proc_self_fd) + ..xattr(). Another related idea is to do > > a unshare(CLONE_FS) after each thread's startup (will pthread library > > balk? I don't know) so that it's safe to do fchdir(lo->proc_self_fd) > > + ...xattr() + fchdir(lo->root_fd). > > Hi Miklos, > > Trying to understand your proposal. So if we want to do an ..xattr() > operation on a special file (and we don't have _at() variants), how > will fchdir() help. Are you suggesting fchdir() to parent and then > do something special. > > Can you please elaborate your proposal a bit more. I think I have > missed the core idea completely. > > I understand that you want to do unshare(CLONE_FS) to make sure one thrad's > fchdir() does not impact other thread. But did not understand that how > fchdir() alone is enough to do getxattr()/setxattr() on special file.
The fchdir() call is to avoid having to do openat(). The openat() was needed because /proc/self/fd/ is only accessible through a file handle (lo->proc_self_fd). Thanks, Miklos
